This invention relates to a novel block copolymer composition for use in adhesives. More particularly, it relates to high molecular weight low coupled linear styrene-isoprene-styrene block copolymer compositions comprised of linear polymeric blocks and adhesives made using such compositions.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU A-B and A-B-A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene such as polyisoprene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. When the content of the alkenyl arene is small, the produced block copolymer is a so-called thermoplastic rubber. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases--a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A) called domains. Since the A-B-A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure.
These domains act as physical crosslinks anchoring the ends of many block copolymer chains. Such a phenomena allows the A-B-A rubber to behave like a conventional vulcanized rubber in the unvulcanized state and is applicable for various uses. For example, these network forming polymers are applicable for uses such as in adhesive formulations; as moldings of shoe soles, etc.; impact modifier for polystyrene resins and engineering thermoplastics; modification of asphalt; etc.
Different linear block copolymers exhibit different performances in adhesive formulations. Therefore, it requires some experimentation to determine the range of polymers which are useful for a particular adhesive application. For instance, it has been found that a linear S-I-S block polymer having a peak molecular weight as measured by gel permeation chromatography of 200 to 220, a polystyrene content of 20 to 25% and a coupling efficiency of 30 to 40% has good adhesive properties, especially peel strength but that its holding power in an adhesive composition could be advantageously increased for certain end uses such as labels with improved die cuttability, tapes, or adhesive bandages. Similarly, it has been found that a similar polymer with a coupling efficiency of 80 to 90% exhibits very good holding power but it would be advantageous to increase its peel strength so it would be more useful in the aforementioned end uses. The polymer and adhesive composition of the present invention provides the advantages of both of these types of polymers and eliminates their disadvantages.